Bob Cordell's Power amplifier book - Page 258 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 7th February 2013, 10:54 AM   #2571
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by wahab View Post
One thing i do not agree with in Bob s book is that it is said that
"parralelling mosfets is not as straightforward as with BJTs".

Might be true for verticals but laterals can be parralelled even easier
than BJTs.

Indeed , the part devoted to laterals is somewhat symbolic ,
it would have been great to give thoses devices a little
more attention.

Perhaps in the next edition?.
Hi wahab,

The paralleling issue for MOSFETs is indeed an interesting one, and much of what you say is true. I think the main reason I said that it was more difficult was because of the relative ineffectiveness of source degeneration in sharing current among MOSFETs that are not matched. The gate threshold voltages of randomly-selected MOSFETs can vary quite a bit, resulting in current hogging in the absence of source resistors. It is true that this is a smaller problem with laterals because they have lower gm and they also have the somewhat self-regulating characteristic in which conduction decreases as they get hotter over much of their operating current range; so one device that is hogging gets hotter and tends to back away from its hogging. This is not the case for verticals in the current ranges where we normally operate them (the TC crossover occurs at a much higher current in verticals).

Nevertheless, even with laterals manufacturers generally must used matched sets of MOSFETs, be they vertical or lateral.

It is desirable to use matched sets of BJTs as well, where they are matched for beta, but this is not as critical a matter for bias stability for BJTs when they are used with a reasonable value of emitter resistor.

I'll try to give more attention to laterals in the next edition. I may even add a chapter or two on tube amplifiers. Many people were disappointed that I did not say anything about tubes, but instead went to the "dark side" in covering class D amplifiers :-).

Cheers,
Bob
  Reply With Quote
Old 7th February 2013, 12:25 PM   #2572
Bonsai is offline Bonsai  Taiwan
diyAudio Member
 
Bonsai's Avatar
 
Join Date: Jul 2003
Location: Taipei, Taiwan
Now we know why so many MOSFET amps only have one pair of output devices . . .
__________________
bonsai
Amplifier Design and Construction for MUSIC! http://hifisonix.com/
  Reply With Quote
Old 7th February 2013, 01:14 PM   #2573
JPV is offline JPV  Belgium
diyAudio Member
 
Join Date: Dec 2006
I have a few comments/questions about this topology.

This is a fully differential amplifier.
To accomplish this two equal loads are placed in the two collectors of the input differential amplifier, the ouput beeing take differentially.
To have enough gain, these two loads should have very high resistance.
In IC design this is done by putting two identical current sources one in each collector. But now we have a biasing problem the current source of the differentail amplifier and the two loading currents sources are both establishing the bias which is not possible.
The trick is to create a signal equal to the common mode and to feed it back to control one of the bias source, in this way there is only one bias controling element. One simple way to do this is to use two identical resistors as in the fig attached. The mid point of the two resistors cancels the differential signal and is proportional to the common mode voltage. This is applied to the bias voltage of the two current sources.
But now we have a voltage gain equal to gm ro R where ro is the output impedance of the loading current sources, the resistor should again be high. In IC technology the resistors are limited. A way to increase them is to insert an emitter follower before each resistors and this is the topology found in the book.

The question I have is : In discrete design we can use very high value resistors. Should we not have essentially the same performance by simply putting two equal and high resistors either as in the figure or simply in each collector.

JPV
Attached Images
File Type: jpg diff ampli.JPG (46.8 KB, 179 views)
  Reply With Quote
Old 7th February 2013, 01:18 PM   #2574
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Found three copies at a Half Price Books in brand new condition.
I've previously lost two to possibly permanent loan-outs, So I
picked up a few spares. Wonder how long these will last me?

Isn't it a little too soon for this book to be found at discount?
Nineteen bucks I gave, couldn't believe it. Half? Not even half...

I didn't hesitate to quibble over which edition.

Last edited by kenpeter; 7th February 2013 at 01:23 PM.
  Reply With Quote
Old 7th February 2013, 01:32 PM   #2575
JPV is offline JPV  Belgium
diyAudio Member
 
Join Date: Dec 2006
The gain should read: gm ro2//r04//R

R is limiting the gain
  Reply With Quote
Old 7th February 2013, 06:56 PM   #2576
Magna is offline Magna  India
diyAudio Member
 
Join Date: Nov 2004
Location: Chennai
Hi Bob,

What's the prefered sensing location of a VBE device, for vertical mosfets (k1530 & j201 pair), if they are matched for 5% Vgs in a typical high power application, perhaps with a +/- 75v supply...? On top of the device or heatsink...? I 've read that mounting the Vbe on heat sink would suffice in chapter 11 (fig 11.8) of your book. Does mounting the device on top of the output devices would over compensate....?
  Reply With Quote
Old 7th February 2013, 07:25 PM   #2577
diyAudio Member
 
Edmond Stuart's Avatar
 
Join Date: Nov 2003
Location: Amsterdam
Default just teasing

The preferred sensing locations are the voltage drops across the source series resistors.

Cheers,
E.
__________________
Een volk dat voor tirannen zwicht, zal meer dan lijf en
goed verliezen dan dooft het licht…(H.M. van Randwijk)
  Reply With Quote
Old 7th February 2013, 09:40 PM   #2578
diyAudio Member
 
Join Date: Nov 2010
Quote:
Originally Posted by Bob Cordell View Post
I'll try to give more attention to laterals in the next edition. I may even add a chapter or two on tube amplifiers. Many people were disappointed that I did not say anything about tubes, but instead went to the "dark side" in covering class D amplifiers :-).
My personal preference is for more information on solid state, performance aspects. If practical limits on book size mean less or no Class-D and tubes then that is fine. There is always a balance between depth and breadth. In this case both topics are rather specialized and self-contained so they would be a minimal loss compared to the benefit of more space for the central focus of the book.

Best wishes
David

Last edited by Dave Zan; 7th February 2013 at 09:43 PM.
  Reply With Quote
Old 7th February 2013, 09:48 PM   #2579
diyAudio Member
 
Join Date: May 2005
Location: Bristol, UK
Quote:
Originally Posted by Dave Zan View Post
If practical limits on book size mean less or no Class-D and tubes then that is fine. … In this case both topics are rather specialized
Class-D is the future. More on Class-D! More on advanced high-order loops and how to make them stable under clipping conditions. Basically tell us all about NCore
  Reply With Quote
Old 7th February 2013, 10:10 PM   #2580
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by Bob Cordell View Post
I'll try to give more attention to laterals in the next edition. I may even add a chapter or two on tube amplifiers. Many people were disappointed that I did not say anything about tubes, but instead went to the "dark side" in covering class D amplifiers :-).

Cheers,
Bob
Definitely more attention to laterals. These days, the SS manufacturers are putting much more emphasis on switching type applications. Hopefully, more attention will make more devices useful for linear amplification more available. It's not good that this has been so thoroughly short changed lately. There are already more than enough sub standard SS amps on the market, and SS doesn't have to sound as horrible as it does all too often. However, sonic excellence still needs devices that can perform well in that application.

"I may even add a chapter or two on tube amplifiers".

Don't. Hollow state will require more than a "couple of chapters" to do it right. Many of the concepts carry over from SS to HS, but the design details are very different. Besides, Morgan Jones has this covered. Maybe more about Class D, for those who're interested?

"Class-D is the future".

Not really, it's been around for quite a few dog's ages now. You can also implement Class D in hollow state, as it was first done back in the early '50s.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Another realization of Bob Cordell's THD Analyzer giulianodes Equipment & Tools 37 9th October 2013 02:48 AM
best audio amplifier book?? Bouvett Everything Else 30 13th August 2012 02:43 AM
Amplifier Design Book pixie Everything Else 27 11th June 2010 08:36 PM
Project 11.1 from Slone "High-Power Amplifier" Book Karl71 Solid State 46 6th October 2008 03:47 AM
book-audio power amplifier by Douglas Self mikee12345 Solid State 8 16th November 2003 01:16 AM


New To Site? Need Help?

All times are GMT. The time now is 11:28 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2